Method for continuous casting of steel through a closed gas filled chamber

ABSTRACT

1,086,094. Continuous casting. UNITED STEEL COMPANIES Ltd. Nov. 19, 1965 [Nov. 24, 1964], No. 47823/64. Heading B3F. In a method of continuous casting in which the molten metal is poured from a tundish 1 into a reciprocating mould 4 means are provided for connecting the tundish to the mould so as to form a chamber through which the molten metal passes and which contains a reducing or an inert gas. The chamber comprises a cooled upper part 13 attached to the tundish and lower part 15 which is attached to the mould and an intermediate part which effects sealing engagement between the ports 13 and 15. The seals 20 permit the axial reciprocation of the mould and the seal 21 permits adjustment of the port 13 transversely of the mould. Prior to the pouring of the metal reducing or inert gas is introduced into the chamber through passage 24 to purge the mould and chamber and is subsequently supplied during the casting operation. The gas pressure during casting is maintained substantially atmospheric by controlling the gas supply through passage 24 and the escape through passage 25 in conjunction with the level of metal in the mould. The choice of gas used in the process depends on the nature of the material being east and include propane, butane, nitrogen etc. or suitable combinations of such gases and whereby oxygen concentration in the chamber is not greater than 250 parts per million. In a further embodiment. Fig. 6 (not shown) part spherical seaitings are provided on the upper and lower ports of the chamber member to facilitate slight angular misalignment of the tundish and the mould for example by tilting of the tundish or by reciprocation of the mould in an arcuate path, when a curved mould is used.

p 4. 1968 M. D. HALLIDAY 3 402,757

I. 9 METHOD FOR CONTINUOUS CASTING OF STEEL THROUGH A CLOSED GAS FILLED CHAMBER Filed Nov. 22, 1965 United States Patent 3,402,757 METHOD FOR CONTINUOUS CASTING OF STEEL THROUGH A CLOSED GAS FILLED CHAMBER Iain M. D. Halliday, Barrow-in-Furness, England, assignors to The United Steel Companies Limited, Sheffield, England, a British company Filed Nov. 22, 1965, Ser. No. 508,895 Claims priority, application Great Britain, Nov. 24, 1964, 47,823/ 64 7 Claims. (Cl. 164-66) ABSTRACT OF THE DISCLOSURE A method for continuous casting of steel through a gas filled chamber obtains a strand substantially free from oxide formed during pouring and from pin holes Without the use of aluminum. In the method the closed chamber is purged with a reducing or inert gas before starting the cast and is then filled with the gas. An increase in pressure in the closed chamber as the casting proceeds is reduced by permitting the gas to escape at a greater rate than it enters, and reducing gas is passed through the chamber at such a rate that oxygen concentration in the chamber is not greater than 250 parts per million while maintaining the pressure in the chamber substantially constant.

This invention relates to improvements in the continuous casting of steel.

In the continuous casting of steel, the metal is commonly poured into a tundish from a ladle and runs from the tundish into an open-ended mould. As the metal stream flows from the ladle to the mould there is a tendency to oxidation of the stream which it is desirable to avoid. This tendency shows itself particularly with chromium-containing steels, aluminium-stabilised steels and other steels containing alloying elements which have a high affinity for oxygen. When carbon and low-alloy steels are cast, there is not so much oxidation but there is still a strong tendency for minute pinholes to form at the surface of the cast metal or immediately below the surface. The mechanism of pinhole formation is not known. The formation of pinholes can, however, be greatly reduced or virtually eliminated by feeding aluminium wire into the metal stream as this enters the mould, but this is of little effect in eliminating oxidation and it is undesirable to make any aluminium additions whatever to some steels.

In studying the origin of inclusions in continuously cast steel, particularly oxide inclusions, we have found that about 90% of the oxide in the cast steel is derived from oxygen pick-up by the metal during pouring between the tundish and the mould. Thus, non-metallic oxide material and any extraneous slag carried over in the metal from the ladle to the tundish tends to separate out at the tundish, only a very small proportion of such material passing through the tundish nozzle.

It is Well known that in the casting of stainless steels it is necessary to surround the pouring stream by reducing gas such as propane so as to prevent the formation of chromium oxides. We have also found that such gas must be used in casting other steels that include elements having a high affinity for oxygen. When aluminium is used to prevent pinhole formation, the formation of aluminium oxide is found to be particularly rapid even with considerable amounts of propane gas blanketing. As a result, the cast product tends to have a very high proportion of alumina particles embedded in its surface, some of these particles being as much as A; to 7 in. in size.

We have found that it is possible to obtain castings substantially free from oxide formed during the pouring and from pinholes without the use of aluminium by pouring the steel from a tundish or other vessel into the mould through a closed chamber if certain steps are employed. These steps, according to the invention, comprise purging the closed chamber with a reducing gas such as propane or butane or with an inert gas such as argon or nitrogen before the start of a cast; permitting metal to flow through the chamber to the mould while the chamber is substantially filled with the reducing or inert gas; as the mould becomes filled with metal with a consequent tendency to substantial increase in the gas pressure in the closed chamber, reducing this pressure, advantageously substantially to atmospheric pressure, by permitting gas to escape at a greater rate than it enters; and as the casting proceeds passing reducing gas through the chamber at a rate such as to ensure that the oxygen concentration in the chamber is not greater than 250 parts per million while maintaining the pressure in the chamber substantially constant.

The important feature is the low concentration of oxygen, and preferably this is maintained at less than parts per million, better still at less than 50 parts per million, and if possible at no more than 20 parts per million. To ensure this the chamber must be truly closed, but even so oxygen tends to be introduced into the chamber with the molten metal, and minor leakages may occur during casting; it is therefore necessary to maintain a continuous flow of reducing gas through the chamber throughout the cast. It is of course also necessary first to purge the chamber.

The choice of the gas used depends to some extent upon the nature of the steel that is being cast. For ordinary carbon and other steels prone to pinhole formation, when the main problem is the prevention of pinholes, propane gas or other reducing gas or gas mixtures is normally preferred, as it is also for steels containing easily oxidised alloy elements such as aluminium, but not prone to hydrogen embrittlement. In the casting of stainless steels and other steels sensitive to hydrogen embrittlement, an inert gas such as argon or nitrogen may be used. In fact, for such steels nitrogen is generally preferred on the grounds of cost, but since commercial nitrogen contains about 200 parts per million of oxygen the concentration of oxygen in the closed chamber may be up to 250 parts per million. Under these conditions some oxidation of the chromium contained in the steels may occur. If this oxidation is to be minimised, the gas that flows through the closed chamber during the casting may be a mixture of an inert gas such as nitrogen and a reducing gas such as propane or other reducing gas in a concentration such that the sum of the hydrogen and water vapour contents does not amount to more than 5,000 parts per million by volume. Again a mixture of nitrogen and a small proportion of carbon monoxide, say up to 5%, may be used.

In the purging of the closed chamber it is naturally desirable to use as cheap a gas as possible, particularly if the purging gas is rfiowing for some time before the start of a cast. Although it is convenient to purge with the gas that is to be passed through the chamber subsequently, it may be more economical to effect the purging with one gas and to change over to another directly the casting begins.

As the mould fills with metal at the beginning of a cast, the pressure rises in the closed chamber. If there be any substantial gas pressure in the closed chamber it hinders the downward flow of the metal, and it is even possible that the gas will bubble upwards through the metal into the metal in the tundish or other pouring vessel and endanger the operators. It is to avoid these effects that when the flow of metal is started the pressure in the chamber must be allowed to fall or at least must not be allowed to rise appreciably above atmospheric. This can be done by increasing the opening of an outlet valve or opening an additional vent valve. It is highly desirable that during the casting the rate of flow of the metal should be substantially constant, and accordingly the pressure in the closed chamber must be maintained substantially constant, since if it varies it will affect the rate of flow.

In practice the moulds used in the continuous casting of steel are nearly always reciprocated in relation to the casting during the operation.

The stroke is normally small, but of course the reciprocation causes the volume of the closed chamber to vary cyclically with consequential variation of the pressure throughout in the same way.

The invention will be described in connection with the accompanying drawing in which:

The figure is a diagrammatic sectional elevation view of a tundish and mold connected by a closed chamber [for practicing the method of this invention.

The closed chamber may take various forms and in designing it account must be taken of the reciprocation of the mould. One simple design is shown diagrammatically in the figure of the accompanying drawings. Here a tundish 1 with a nozzle 2 let into its base carries a fitting 3 on its underside. The mould is shown at 4, and a fitting 5 is united to the top of it in air-tight fashion. The fittings 3 and 5 are interconnected by a ring 6 of flexible material such as asbestos fabric impregnated with rubber to render it impermeable to gas. The edges of this rings are tightly sealed in the fittings 3 and 5. Alternatively the flexible ring 6 may be made of thin sheet metal resistant to the heat to which it is subjected.

The chamber thus formed within the fittings 3 and S and the ring 6 has a gas inlet 7 to which a valve-controlled pipe for the supply of gas is connected, a gas outlet 8, and a valve-controlled vent 9. In addition there is a silica window through which the metal stream may be observed.

A long length of pipe 1-1 which may be from A2 to 1 /2 inch in diameter conveys away the efiluent gas from the chamber. The length of this pipe 11 should be sufiicient to ensure that the increase in volume when the mould makes a down stroke cannot result in atmospheric oxygen being sucked into the closed chamber. This increase in volume during a mould downstroke is compensated for to some extent by the supply of gas to the chamber, but at high reciprocation rates it is still necessary to provide some length of pipe '11. Preferably a valve is included in the pipe .11 to aid control of the gas pressure 'within the enclosure.

In operation, metal poured into the tundish from a ladle, to flow through the nozzle 2, is of such a design as to produce a rod-like stream. Before the pouring of the metal begins, the chamber is purged with gas supplied through the inlet 7. This gas passes out not only through the outlet 8 and pipe 11, but also through the nozzle 2. When pouring begins, the metal flows from the tundish through the nozzle 2 as a rod-like stream through the closed chamber into the mould, the base of which is initially closed by a dummy bar. As the metal begins to rise in the mould, the pressure in the chamber begins to rise, and the vent 9 is opened. When the metal reaches a predetermined level in the mould, the dummy bar is lowered so that the strand begins to form and is continuously withdrawn, and this vent 9 is closed again.

The gas outlet from the closed chamber, or the additional vent if one is provided, may be closed under automatic control after being opened at the start of a cast. Thus the valve in the outlet or in the pipe attached to .4 the outlet may be moved to a set postion in response to a signal derived from the level of the metal in the mould, or the additional vent may be closed by such a signal. The signal may be given, for example, by a detector of the gamma ray type. g

The gas pressure in the closed chamber. duringthe casting may be maintained substantially constant under automatic control in that a valve in the gas inlet, or preferably in the gas outlet, may be controllediby a signal given by the pressure in the chamber and smoothed to prevent the pulsations caused by the reciprocation of the mould from having any effect.

I claim:

-1. In the continuous casting of steel by pouring the steel from a tundish or other vessel into a mould through a closed chamber, a method of obtaining a strand substantially free from oxide formed during the pouring and from pinholes Without the use of aluminum which comprises the steps of purging the closed chamber with a reducing or inert gas before the start of a cast; permitting metal to flow through the chamber to the mold while the chamber is substantially filled with the reducing or inert gas; as the mold becomes filled with metal with a consequent tendency to substantial increase in the gas pressure in the closed chamber, reducing this pressure to a pressure insuflicient to hinder the downward flow of metal by permitting gas to escape at a greater rate than it enters; maintaining the pressure of the gas in the closed chamber substantially constant during the casting so as not to affect the rate of metal flow, and as the casting proceeds passing reducing gas through the chamber at a rate such as to ensure that the oxygen concentration in the chamber is not greater than 250 parts per million while maintaining the pressure in the chamber substantially constant.

2. A method according to claim 1 in which the steel cast is carbon steel and the purging gas is a reducing gas.

3. A method according to claim 1 in which the steel cast is stainless steel, and the gas is an inert gas and a reducing gas in a concentration such that the sum of the hydrogen and water vapour contents does not amount to more than 5,000 parts per million by volume.

4. A method according to claim :1 in which the pressure is reduced after the purging substantially to atmospheric pressure.

'5. A method according to claim 1 in which the oxygen concentration during the casting is maintained at less than .100 parts per million.

6. A method according to claim 1 in which the reduction in pressure is initiated by a signal derived from the level of the metal in the mould.

7. A method according to claim .1 in which the steel cast is stainless steel and the gas is a mixture of nitrogen r and a small proportion of carbon monoxide.

References Cited UNITED STATES PATENTS 3,268,958 8/ 1966 Sickbert l6464 X 2,825,104 3/1958 Jones l64--281 XR 2,889,596 6/1'959 Savage et a1. l64155 3,125,440 3/1964 Hornak et al l6464 XR 3,310,850 3/1967 Armbruster 16464 0 J. SPENCER OVERHOLSER, Primary Examiner.

R. S. ANNEAR, Assistant Examiner. 

